Embodiments of the present invention is directed to methods and apparatus for a signal modulator driver. Some embodiments of the invention are directed to methods and apparatus for realization of a PAM (Pulse Amplitude Modulation) modulator driver providing electrically adjustable pre-distortion of the PAM output waveform.
Pulse-amplitude modulation (PAM) is widely used in modulating signal transmission of digital data. For example, PAM4 (Pulse Amplitude Modulation-4) is a modulation technique in which four distinct pulse amplitudes are used to convey the information, as shown in FIG. 1. Four amplitude levels can be represented by two binary bits 00, 01, 11, and 10, respectively. Each pair of bits is called a symbol. When one of the four amplitudes is transmitted in a symbol period T, there are two bits transmitted in parallel, therefore the data rate is doubled. In other words, PAM-4 modulation is twice as bandwidth-efficient as conventional binary modulation.
Conventional PAM4 modulator driver architectures used commonly in the art utilize a highly linear wide band power amplifier to amplify a 4-level PAM4 input signal and provide a PAM4 output signal with sufficient amplitude to drive an optical modulator in a communications system with low harmonic distortion. The inventors have identified many drawbacks in the conventional approaches. For example, the conventional approach suffers from high power dissipation due to the requirement of providing high linearity and low distortion of the input signal. It also suffers from PAM4 output eye closure, overshoot, and undershoot due to the non-ideal amplitude and group delay response in the amplifier transfer characteristic, as well as rise and fall time degradation due to bandwidth limitations.
From the above, it is seen that techniques for improved PAM4 modulator driver are highly desired.